1. Field of the Invention
The present invention relates to an abrasive composition. More particularly, the present invention relates to an abrasive composition for rapidly polishing and providing a mirror finish to an aluminum-based substrate for a magnetic recording disc, and a process therefor. This abrasive composition has a high polishing efficiency, is not harmful to the human body such as an operator's hands, etc., and has little corrosive affect on machinery such as a polishing machine and peripheral equipments, and waste therefrom is easily treated.
2. Description of the Related Art
A magnetic recording disc comprises an aluminum-based substrate including an aluminum or aluminum alloy (referred to only as aluminum hereafter) disc, a nickel substrate formed by plating nickel-phosphine onto an aluminum disc, and an anodized aluminum substrate formed by anodizing an aluminum disc. A magnetic film is applied to the aluminum-based substrate by coating, plating or sputtering, and the surface of the aluminum-based substrate is provided with a mirror-finish by polishing.
A known abrasive composition used for polishing an aluminum substrate for a magnetic recording disc, particularly a nickel substrate or an anodized substrate as mentioned above, is a weak acid slurry which comprises water, an alumineous abrasive and a polishing accelerator of nickel sulfate, the composition having a pH of 4 to 6 (Japanese Unexamined Patent Publication (Kokai) No. 61-278587, published on Sept. 12, 1986). The inventors has proposed a similar weak acid abrasive composition in which the polishing accelerator of nickel sulfate is replaced by nickel sulfanate (Japanese Patent Application No. 62-25359, filed on Oct. 9, 1987). Also known is a strong acid slurry which comprises water, an alumineous abrasive and a polishing accelerator of aluminum nitrate, the composition having a pH of 3 to 4 (Japanese Unexamined Patent Publication (Kokai) No. 62-25187, published on Feb. 3, 1988).
A strong acid abrasive composition, in comparison with a weak acid abrasive composition, has a higher polishing efficiency and provides a superior surface finish, but brings problems in that it is corrosive to a polishing machine or peripheral equipment, resulting in wastage or damage thereof and contamination by rust during the polishing operation, and that it may splash onto the skin or clothes of operators and is harmful to the human body, for example, roughens the bands. These effects are caused by nitrate radical (ion) contained in aluminum nitrate, which is highly corrosive to machining and roughens the skin of the human body by breaking bridges of a three dimensional structure of proteins, and which requires a cumbersome treating of dissolved nitrogen in waste water.
To avoid these problems, weak acid abrasive compositions are used, but these are disadvantageous due to a relatively low polishing efficiency and a relatively low grade surface finish.
Therefore, there is a strong demand for an abrasive composition having a polishing efficiency equal to or higher than that obtainable by a strong acid abrasive composition, does not cause a corrosion of machinery and roughening of the skin, and is easily treated as a waste.
The object of the invention is to provide such an abrasive composition.